Sorry there are no clear & easy answers I know of for post-nasal drip.
Post-nasal drip can be cause
d by many different things--do you have allergies? It can be cause
d by allergies. It can also be cause
d by a viral or bacterial infection. When the mucous is yellow, it may be an infection. In any case, I would suggest discussing it with your doctor, particularly if it occurs frequently and often leads to bronchitis.